The present invention is situated in the domain of exhaust gas treatment of automotive vehicles, specifically exhaust gas treatment systems using a reducing agent injected in the exhaust line of a vehicle.
This invention relates, more specifically, to a method intended for implementation in automotive vehicles equipped with a treatment system for nitrogen oxide emitted by a diesel engine, in which the treatment takes place through selective catalytic reduction of nitrogen oxide by means of a reducing agent such as ammonia.
These treatment systems treat the exhaust gas in such a manner that the vehicles meet the legally tolerated emission levels, which are continuously lowered.
The present invention finds an advantageous, but not exclusive, application in vehicles equipped with the type of treatment systems in which the reducing agent, such as ammonia, is stored in aqueous solution in a reservoir, for instance in the form of liquid urea.
The reduction of nitrogen oxide by ammonia through a selective catalytic reduction reaction is a widely used method in stationary factories. The difficulty of transposing this kind of processes in the automotive domain resides in the storage of the reducing agent in the vehicle.
Several solutions have been developed for storing a reducing agent such as ammonia in an automotive vehicle. Solutions were proposed for storing the ammonia in the form of solid urea, liquid urea or ammonium carbamate.
Selective catalytic reduction systems, called SCR, comprise in general a catalyst, the site of one or more chemical reduction reactions of nitrogen oxide by ammonia, and are designated in what follows by the term SCR catalyst.
It was observed that for more efficient treatment of nitrogen oxides, it was helpful to store the ammonia directly inside the SCR catalyst, the site of the chemical reduction. However, the size of the SCR catalyst does not permit storing a sufficient quantity of reducing agent to guarantee an acceptable autonomy of the vehicle. Consequently, in existing systems, a reducing agent reservoir is used which is separate from the SCR catalyst, and the injection of reducing agent in the catalyst is regulated, with an eye on storage, starting from a mathematical model with the quantity of nitrogen oxide emitted by the engine as one of the inputs.
It was however observed that the storage characteristics of the catalyst vary over the lifetime of a vehicle, in particular on account of the reduced storage capacity with age. Consequently, it appeared necessary to permanently control the injection of reducing agent in the catalyst.
Initially, methods were envisaged in which the injection was instantly modified by increasing or decreasing the calculated quantity by means of the mathematical model. To this end, the nature/composition of the emitted gas flowing out of the catalyst is determined by means of a nitrogen oxide sensor. If the emitted gas at the outlet is nitrogen oxide, this means that the injected quantity of reducing agent is insufficient to reduce all nitrogen oxide and if, on the contrary, the emitted gas is a reducing agent such as ammonia, this means that too much reducing agent was injected upstream of the catalyst.
However, it is also possible that the observed over-injections and under-injections are not due to aging of the catalyst, but to an error in the mathematical model for determining the quantity of reducing agent to be injected in the system.